1. Field of Invention
This invention relates to batteries, in particular to large capacity secondary batteries comprising multiple cells.
2. Description of Related Art
A secondary battery is a rechargeable battery, whereas a primary battery is a battery that cannot be recharged. A lithium-ion secondary battery exhibits high voltage, high capacity, and low self-discharge, and is popular source of portable electrical energy. However compared with other types of secondary batteries, a lithium-ion battery poses greater safety hazard. Therefore constant monitoring and control of its current, voltage, temperature, and some other parameters under a battery management unit (BMU) are often required.
A cell is a basic electrochemical unit capable of storing electrical energy, but not necessarily resilient against handling and environmental stresses. A battery comprises at least one cell, and is physically resilient enough to survive reasonable handling. There is a significant overlap between these two definitions. In literature and in this disclosure, the word “battery” and the word “cell” are often used interchangeably.
Large capacity batteries in general comprise multiple cells connected in parallel to deliver the required current and in series to deliver the required voltage. In the simplest configuration, a plurality of cells are connected in series such as can be found in a typical flashlight. If more current is required at a particular voltage, an additional series of cells are connected in parallel to a first set of series connected cells.
Shown in FIG. 1A is a series-parallel connected battery scheme of prior art, where several columns 11 of series connected cells 10 are connected in parallel between a plus voltage output terminal 12 and a minus voltage output terminal 13. The number of cells in a column establishes the required voltage and the number of columns connected in parallel provide the required current. The series-parallel configuration ensures that current except for leakage current is identical throughout each column. Therefore, current distribution within the battery can be monitored and manipulated per column, and not per cell. When a short circuit develops in any column, inrush of current from other columns is limited by the internal resistance of all cells not containing the short. However in this configuration, the voltage of each cell needs to be monitored and balanced separately.
An alternative parallel-series scheme of prior art is shown in FIG. 1B, where a number of cells 10 are connected in parallel to form a row 15, and to provide the required current. Then a number of rows 15 are connected in series to provide the required voltage between the plus voltage output terminal and the minus output voltage terminal. In a parallel-series matrix of lithium-ion rechargeable cells, the voltage of each row needs to be monitored and balanced separately. Uniform current is not insured among parallel cells in any row 15, wherein, for instance, cell impedance can vary with local temperature. When a short-circuit develops in one of the cells, inrush of current from parallel cells is limited only by internal resistance of cells in a single row. Thus the inrush current is greater than that with a series-parallel configuration shown in FIG. 1A.
By elementary physics, current is inherently identical throughout each isolated series, while voltage is inherently identical across each parallel connected row. A series-parallel configuration and a parallel-series configuration each takes advantage of one of the above two laws of physics, to simplify current and voltage control, respectively. However neither configuration is able to simplify both current and voltage controls. A series-parallel configuration requires voltage control for each cell, while a parallel-series configuration suffers from the lack of current control for each cell. In a third configuration (not shown) voltage and current of each cell in a matrix of batteries are independently monitored and controlled providing a complex scheme. In addition more energy is dissipated as a result of the number of controllers that are needed.
U.S. Pat. No. 7,459,882 B2 (Morgan) is directed to an improved rechargeable battery, which comprises a plurality of battery cells connected together to discharge in parallel each of the cells, or each group, or groups of cells that have their own respective recharging input as well as a switching circuit to connect the cells or groups of cells to a charging input. In U.S. Pat. No. 7,394,225 B2 (Guang et al.) a multiple cell battery charger is directed to being configured in a parallel configuration to provide constant current charging. U.S. Pat. No. 7,276,881 B2 (Okumura et al.) is directed to a method of protection for preventing battery cells from over discharge and over charge. U.S. Pat. No. 6,777,908 B2 (Thorne et Al.) is directed to a battery cell balancing method and apparatus to balance cells within a battery where at some of the cells are arranged in series or a combination of series and parallel. U.S. Pat. No. 6,735,098 B2 (Hussein et al.) is directed to an inrush current limiting circuit, a power source device and a power conversion device. U.S. Pat. No. 6,417,646 B1 (Huykoman et al.) is directed to a circuit to protect individual cells of a multi-cell battery from overcharge and to collect data to determine of the cell state of health. In U.S. Pat. No. 6,160,375 (Horie et al.) a series arrangement of a plurality of lithium ion cells is directed to uniform cell charge, wherein a Zener diode and a resistor are connected in series between the positive and negative terminal of each cell to make uniform cell charge when a positive electrode crystal phase begins. U.S. Pat. No. 6,150,795 (Kutkut et al.) is directed to a battery charge equalization that is performed by modules in a staggered means between pairs of batteries in a series connected string of batteries. U.S. Pat. No. 6,114,835 (Price) is directed to a charge balancing circuit, which determines when to initiate a charge balance mode to equalize charge in at least two cells of a multi-cell battery.
U.S. Pat. No. 6,043,628 (Perelle et al.) is directed to a method and control for cells connected in series, wherein each cell is associated with a bypass that is activated by the controls and resulting from measurements in order to balance charge and discharge. In U.S. Pat. No. 5,956,241 (LaCascio) a battery power circuit comprising a cell equalization circuit is directed to insuring each cell in a multiple battery cell stack is depleted of charge at an equal rate. U.S. Pat. No. 5,821,733 (Turnbull) is directed to a system of charging for a multiple series connected battery cells and includes a plurality of shunt regulators. U.S. Pat. No. 5,773,159 (Beard) is directed to a battery pack comprising multiple lithium cells connected in series where a voltage miss match between the lithium cells utilizing circuitry contained within the battery pack. U.S. Pat. No. 5,666,040 (Bourbeau) is directed to a battery monitor and control system where electronic modules are connected to terminals of batteries connected in series to control over-voltage, under-voltage, over-temperature and float-voltage. U.S. Pat. No. 5,650,240 (Rogers) is directed to a multi-cell battery system for a battery of at least two cells, wherein a selective bypass can be selected for each of the cells of the multi-cell battery. U.S. Pat. No. 5,578,914 (Morita) is directed to a battery charging system, which is arranged to reduce a bypass capacity to enable a high current charging operation. U.S. Pat. No. 5,206,578 (Nor) is directed to a monitoring circuit for batteries while being charged, wherein the batteries comprise a series connection of cells to prevent damage to the cells. In U.S. Pat. No. 4,061,955 (Thomas et al.) a multi-cell battery system is disclosed where each cell has an individual protective circuit and wherein each cell is monitored for over charge and undercharge and a bypass circuit is used to remove a bad cell from the series connection of cells. U.S. Pat. No. 3,872,457 (Ray et al.) is directed to a self-monitoring system for batteries, which scans individual battery cells to detect faulty cells.
There exists a need, addressed by this invention, for a battery connection scheme that simplifies the monitor and control of both current and voltage. More specifically a novel scheme that ensures current uniformity, reduces inrush current, simplifies battery cell balancing and facilitates short detection is desired.